This paper investigates the manufacturability limits of fin aspect ratios within two-fluid counter-flow microchannel arrays based on the stress state between laminae during diffusion bonding. In prior papers, it has been shown that the diffusion bonding of two-fluid systems by microlamination can result in regions of the device that do not directly transmit bonding pressure and, consequently, result in unbonded regions leading to device leakage. A finite element model is used to analyze the stress state between laminae during diffusion bonding. The stress state is used to determine the critical stress necessary for diffusion bonding to occur in areas not receiving direct bonding pressure. Model results are compared with experimental results over a wide range of counter-flow geometries. It has been found generally that a compressive stress state must exist in every part of the geometry in order to produce leak-free bonds. Implications of this finding on the design of two-fluid microchannel arrays are discussed.